Performance improvements and refactor of Ratelimiter (#7595)

While working on https://github.com/matrix-org/synapse/issues/5665 I found myself digging into the `Ratelimiter` class and seeing that it was both:

* Rather undocumented, and
* causing a *lot* of config checks

This PR attempts to refactor and comment the `Ratelimiter` class, as well as encourage config file accesses to only be done at instantiation. 

Best to be reviewed commit-by-commit.

1 files changed, 39 insertions, 10 deletions

diff --git a/tests/rest/client/v1/test_login.py b/tests/rest/client/v1/test_login.py
index 0f0f7ca72d..9033f09fd2 100644
--- a/tests/rest/client/v1/test_login.py
+++ b/tests/rest/client/v1/test_login.py
@@ -29,7 +29,6 @@ class LoginRestServletTestCase(unittest.HomeserverTestCase):
     ]
 
     def make_homeserver(self, reactor, clock):
-
         self.hs = self.setup_test_homeserver()
         self.hs.config.enable_registration = True
         self.hs.config.registrations_require_3pid = []
@@ -38,10 +37,20 @@ class LoginRestServletTestCase(unittest.HomeserverTestCase):
 
         return self.hs
 
+    @override_config(
+        {
+            "rc_login": {
+                "address": {"per_second": 0.17, "burst_count": 5},
+                # Prevent the account login ratelimiter from raising first
+                #
+                # This is normally covered by the default test homeserver config
+                # which sets these values to 10000, but as we're overriding the entire
+                # rc_login dict here, we need to set this manually as well
+                "account": {"per_second": 10000, "burst_count": 10000},
+            }
+        }
+    )
     def test_POST_ratelimiting_per_address(self):
-        self.hs.config.rc_login_address.burst_count = 5
-        self.hs.config.rc_login_address.per_second = 0.17
-
         # Create different users so we're sure not to be bothered by the per-user
         # ratelimiter.
         for i in range(0, 6):
@@ -80,10 +89,20 @@ class LoginRestServletTestCase(unittest.HomeserverTestCase):
 
         self.assertEquals(channel.result["code"], b"200", channel.result)
 
+    @override_config(
+        {
+            "rc_login": {
+                "account": {"per_second": 0.17, "burst_count": 5},
+                # Prevent the address login ratelimiter from raising first
+                #
+                # This is normally covered by the default test homeserver config
+                # which sets these values to 10000, but as we're overriding the entire
+                # rc_login dict here, we need to set this manually as well
+                "address": {"per_second": 10000, "burst_count": 10000},
+            }
+        }
+    )
     def test_POST_ratelimiting_per_account(self):
-        self.hs.config.rc_login_account.burst_count = 5
-        self.hs.config.rc_login_account.per_second = 0.17
-
         self.register_user("kermit", "monkey")
 
         for i in range(0, 6):
@@ -119,10 +138,20 @@ class LoginRestServletTestCase(unittest.HomeserverTestCase):
 
         self.assertEquals(channel.result["code"], b"200", channel.result)
 
+    @override_config(
+        {
+            "rc_login": {
+                # Prevent the address login ratelimiter from raising first
+                #
+                # This is normally covered by the default test homeserver config
+                # which sets these values to 10000, but as we're overriding the entire
+                # rc_login dict here, we need to set this manually as well
+                "address": {"per_second": 10000, "burst_count": 10000},
+                "failed_attempts": {"per_second": 0.17, "burst_count": 5},
+            }
+        }
+    )
     def test_POST_ratelimiting_per_account_failed_attempts(self):
-        self.hs.config.rc_login_failed_attempts.burst_count = 5
-        self.hs.config.rc_login_failed_attempts.per_second = 0.17
-
         self.register_user("kermit", "monkey")
 
         for i in range(0, 6):